12.2. Creating and activating snapshot objects

Creating and activating a snapshot is a two-step process. The first step is to create the snapshot object. The snapshot object specifies where the saved data will be stored when changes are made to the original. The second step is to activate the object, which is to make an EVMS volume from the object.

12.2.1. Creating a snapshot

You can create a snapshot object from any unused storage object in EVMS (disks, segments, regions, or feature objects). The size of this consumed object is the size available to the snapshot object. The snapshot object can be smaller or larger than the original volume. If the object is smaller, the original volume could fill up as data is copied from the original to the snapshot, given sufficient activity on the original. In this situation, the snapshot is deactivated and additional I/O to the snapshot fails.

Base the size of the snapshot object on the amount of activity that is likely to take place on the original during the lifetime of the snapshot. The more changes that occur on the original and the longer the snapshot is expected to remain active, the larger the snapshot should be. Clearly, determining this calculation is not simple and requires trial and error to determine the correct snapshot object size to use for a particular situation. The goal is to create a snapshot object large enough to prevent the shapshot from being deactivated if it fills up, yet small enough to not waste disk space. If the snapshot object is the same size as the original volume (actually, a little larger, to account for the snapshot mapping tables), the snapshot is never deactivated.

12.2.2. Activating a snapshot

After you create a snapshot, activate it by making an EVMS volume from the object. After you create the volume and save the changes, the snapshot is active. The only option for activating snapshots is the name to give the EVMS volume. This name can be the same as or different than the name of the snapshot object.